CMOS image sensors can be easily manufactured at a low cost as compared with Charge Coupled Devices (CCDs), and thus are among the most commonly used image pickup devices. Also, because a unit pixel of a CMOS image sensor is formed of MOS transistors, a CMOS image sensor can be embodied within a narrower area than that of a CCD thereby providing higher resolution. Furthermore, signal processing logic can be formed in an image circuit where the pixels are formed, and the image circuit and the signal processing circuit can be integrated into a single circuit.
Such CMOS image sensors are formed of a plurality of unit pixels to achieve high resolution. Each unit pixel includes a photodiode for sensing images, and transistors (e.g., a transfer transistor, a reset transistor, and a drive transistor (or source follower)), to transfer a signal sensed by the photodiode to a signal processing circuit. CMOS image sensors can be used in diverse mobile electronic products such as cellular phones, Personal Digital Assistants (PDAs), and digital cameras. Since devices such as PDAs and cellular phones are being reduced in size, a demand for higher integration of the CMOS image sensors is increasing. Since a unit pixel size has to be reduced in order to achieve higher integration of a CMOS image sensor, areas of the photodiode and the transistors constituting the unit pixel are being gradually decreased.
However, if a transistor area is decreased as stated above, a flicker noise phenomenon can occur on a screen that outputs a captured image. It is known that flicker noise occurs due to partially trapped charge on an interfacial surface of a substrate (silicon) and a gate oxide layer when passing through a channel region of the transistor. In particular, experiments have confirmed that flicker noise becomes further sensitive to a decrease in an area of a gate insulating layer (i.e., a channel length), and is most affected by a gate insulating layer area of a source follower SF (i.e., a drive transistor that amplifies charge stored in a floating diffusion region FD to form a signal).
To solve these problems, a method of increasing a gate area of the drive transistor has been suggested. However, if the gate area of the drive transistor is increased, a fill factor is decreased, which may degrade a picture quality characteristic of the CMOS image sensor. Therefore, a method of increasing an area of a gate insulating layer of a drive transistor within a range that does not reduce the fill factor is required.